How far can you go with interior mods?

[crankit]

Does anyone know what the limits are to modifying a factory interior without having to get engineering certificates or the car being un-roadworthy?

I'm good with fiberglass and wood from boat building and I was thinking of making my own center console and maybe modifying the dash of my old column shift ute. Possibly even going so far as to redo the hole car including custom interior.